Disorders of balance, especially when they result in falls, are an important and common clinical problem. The cause of disequilibrium in many instances is dysfunction of the vestibulospinal reflexes but most clinical vestibular research has focused on the vestibulo-ocular reflex (VOR). Although much is known about the adaptive strategies used by labyrinthine- deficient humans to stabilize gaze, almost nothing is known about the adaptive strategies to stabilize posture. Accordingly, the long-term objective of our research is to understand better the adaptive capabilities of postural stabilizing reflexes (PSR) with the ultimate goal to improve the physical therapy treatment of patients with balance disorders. By studying both normal subjects and patients without labyrinthine function, we will develop testing paradigms and analysis techniques for PSR adaptation that are comparable to those that have been successfully used to study VOR adaptation. PSR adaptation will be studied by measuring 1) The amplitude of sway induced by a sudden perturbation of the support surface and 2) The amplitude of sway induced in response to a moving visual stimulus. These two approaches are of particular interest because they simulate conditions in which patients with labyrinthine deficits have complaints of imbalance. Our main approach will be to quantify fore-aft sway under conditions of artificial feedback to isolate the various sensory components that contribute to the postural stabilizing reflexes. To induce adaptation, subjects will wear reversing (dove) prisms or magnifying (X2) spectacles, both of which change the apparent motion of the environment during head rotation. We will compare adaptation of the VOR with that of PSR. The results of the test in normal subjects will be compared with the results from subjects with bilateral labyrinthine deficiencies. The results of this project will be applied to the design and the assessment of physical therapy programs for patients with balance disorders.